Old Phone Intercom Device

This Photo Instructable shows how to use two old phones as a cheap intercom between Point A and Point B.  Might be useful between the house and the shop, between the treehouse and the fort, between your kids' rooms, etc.

This was originally build for my grade-school girls to talk to each other on the telephone between their bedrooms.

Basically it is a two-jack phone wall plate modified to act as the connecting point for two extension phones.  The plate ties the two 'lines' together and supplies power.  Two phones are connected to their respective jacks.  Taking the phones 'off hook' will allow the batteries to power the circuit and you can carry on a conversation.

The batteries are connected in series to provide 18V to power the phones.  The battery life is not that great as I recall, so it may be worthwhile to investigate using an old laptop plug-in power supply to power it.  I understand that some very old phones may have polarity sensitivity so if the circuit does not work try reversing the battery connection to the wall jacks.

The schematic shows the connections.  Don't forget the 300 ohm resistor as shown.  A call buzzer or light could be added with additional components and wiring to make the setup more useful -- but for two pre-teens between bedrooms this was sufficient.

Enjoy!

Comments

[muddog15]

why do we need the resistor

[blkhawk]

Instead of electrician's black tape you could use an electrical box to enclose the batteries. Some boxes are made of plastic if you do not want to use the metal one.

[wannabemadsci]

Thanks for the suggestion. It would make it much nicer to use a plastic electrical box over the back since that is what the jack plate is intended to mount to.

[blkhawk]

You have inspired me to make this project. I used a dead laptop power supply to power the phones.

[rimar2000]

This is just what I am needing! Thanks, I'll try to do it.

[francisdsa]

check this one out too...
http://www.instructables.com/id/CAT5-Intercom/

[rimar2000]

That is more complicated! I am too lazy for make it...

Thanks anyway.

[Kozz]

I think these are great! I always wanted to build something like these for my boys. I understand that there is some kind of a voltage spike (? I'm not an electronics guy) needed to induce a "ring". It would be so cool to figure out how to modify the circuit to allow the phones to ring, too. :)

[francisdsa]

you can check out a similar circuitry i did with a ringer and using a common DC power adapter. here is the link
http://www.instructables.com/id/CAT5-Intercom/

[wannabemadsci]

I understand that true ringing voltage on a 'regular' phone line is 90VAC at 20Hz. Much easier and safer to add some additional wires or utilize the yellow and black wires along with one or both sides of the 18V supply to add a buzzer or other indicator that there is a call coming in. I'll have to put some thought into it. Thanks for the comment.

[ilpug]

I want to do this, but to conserve battery life I will put a switch on the battery plate to turn it off when not in use.

[wannabemadsci]

Great idea. Thanks for the comment.